Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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075 FXUS61 KPHI 230726 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 326 AM EDT Sun Jul 23 2017 .SYNOPSIS... A frontal boundary extended from the southern Great Lakes to our region this afternoon. Low pressure will ride eastward along the front and it should pass through our area tonight. Another low is expected to follow on Monday. The second low is forecast to pull the boundary southward and away from our region allowing high pressure to build down from the northwest for the middle part of the new week. A cold front from the northwest is anticipated to arrive on Thursday night. && .NEAR TERM /THROUGH TODAY/... Quite a bit to talk about in the near term. Currently, MCS is slowly propagating southward/southeastward off the coast of Delmarva, but cells continue to develop via upwind propagation processes on the southwest flank. A large shield of lighter precipitation has developed to the north of the main convective band, still providing a nice steady rain and anvil lightning to far southern portions of the CWA. Meanwhile, showers continue developing on the west side of Chesapeake Bay, aided by convectively-induced perturbations moving through the southern Mid-Atlantic region at this time. Have prolonged PoPs in this region through the overnight hours to account for these trends. Farther north, long-lived MCV is moving offshore south of Long Island, which provided a fairly sizable swath of 0.5-2 inch(es) of rain tonight from Lehigh County eastward through Hunterdon/Somerset/Middlesex/Monmouth Counties in New Jersey. This will be an important consideration with regards to tonight`s forecast (see next section). The aforementioned perturbations will be moving offshore this morning, and with convective overturning affecting much of the area, see very little in the way of precipitation chances later this morning. Cloud cover is going to hang tough, I suspect, owing to some northeasterly flow that will seep into the region. KDIX is indicating an outflow boundary from the aforementioned MCV that is sagging slowly southward through central New Jersey (as of 230 am located generally from Trenton to Manasquan. Winds become northeasterly to the north of this boundary, and dew points are a few degrees lower. Higher-resolution model output suggests this may continue southward to approximately the Mason- Dixon Line through a considerable portion of the day (at least a chunk of the morning hours) - in addition, some showers may develop along this boundary (as seen in the past hour). Models are notoriously too eager with dissipating such boundaries or returning them northward. As such, I bumped down dew points considerably (around 3-5 degrees) north of the Mason-Dixon Line today and re-oriented the winds to a more northeasterly direction for a longer duration for these areas as well. Guidance is noticeably cooler with highs today, and this makes sense given the current observational trends. Will be interesting to see how much clearing, if any, occurs today. Regarding convective potential, the next in a series of midlevel perturbations moves toward the region this afternoon and tonight, and with somewhat more focused large-scale ascent downstream in a weakly to negligibly capped environment, scattered to widespread convection should develop this afternoon to our west. The timing of its approach to our region looks rather late in the day, and held off on higher PoPs to mostly the tonight period. However, some storms may move into the far western CWA by late afternoon. With the approach of stronger westerlies as a larger-scale trough begins to amplify today, vertical shear profiles will continue to improve. Think the main question today will be degree of instability, with remnant cloud cover and outflow boundaries likely making model depictions of the thermodynamic environment mere pipe dreams. However, southwesterly flow to the south/west of the area should advect warm/moist air northeastward, allowing boundary- layer based parcels to attain CAPE values 1000-2000 J/kg by afternoon near/south of the convectively-reinforced surface boundary in the southern Mid-Atlantic. Where this boundary ends up is anyone`s guess (mine is south of consensus, based on the past 24 hours of analysis). Should such instability materialize, CAPE-shear parameter space is favorable for organized severe, with downburst winds associated with bowing segments and/or mesovortices embedded within convective segments/clusters the main threat. Highly heterogeneous convective environment would suggest that more organized (lengthier) lines of convection may be hard to materialize, especially given the very moist boundary layer hindering the development of somewhat stronger cold pools. CAM guidance hints at such a scenario playing out, and given pattern recognition and the past 24 hours as "analog guidance", this increases confidence in at least the overall mode of strong to severe convection this afternoon/evening - again, should the stronger instability pan out. With PWs 1.75+ inches (somewhat lower than the past 24 hours, aided by somewhat drier midlevels and a southward push of the convectively-induced surface boundary) and the potential for strong updrafts should near-surface based instability materialize, heavy rainfall is a threat with any of the stronger storms today. && .SHORT TERM /TONIGHT/... Tonight`s forecast could be metaphorically described as throwing darts in the dark. As convection from the west moves into the region overnight, there is a clear signal from CAM guidance of developing a mesoscale convective vortex on the north side of the convection, with larger-scale influence via a developing/eastward-moving surface low translating across the Mid-Atlantic during this period (as the trough in the eastern Ohio Valley continues to amplify). Some CAM depictions (and even some of the coarser guidance) produce very alarming QPF during this period (with 3 to 6+ inches indicated in a corridor generally between I-78 and I-80) as the MCV trudges eastward. This seems way overdone, and the CAMs have had a history of that in the past 24 hours. The concern is that the thermodynamic support just will not be there (or, alternatively, will set up farther south) since the convection occurring the past few hours has displaced the surface boundary to far southern Virginia at this time. My suspicion is that the higher side of the QPF distribution is a low probability, but I suspect there will be a swath of heavy rainfall totals (maybe 1-3 inches rather than 3-6) where convective organization combines with large-scale support. Regarding the latter, as the trough to the west amplifies, a downstream 250-mb jet streak in New England provides substantial upper-level divergence. Considerable warm/moist air will be lofted above the zonally- oriented convectively-reinforced boundary ... somewhere ... in the Mid-Atlantic region and advected northward. Meanwhile, with antecedent heavy rainfall in portions of the region (the Lehigh to Monmouth County corridor, for instance), ingredients are in play for a flood event. Too much uncertainty for a watch at this time, but flooding potential is definitely a concern tonight. Stay tuned. Temp forecast is a blend of MET/MAV MOS with some addition of CAM 2-meter temperatures to account for effects from precipitation. && .LONG TERM /MONDAY THROUGH SATURDAY/... Discussion to be updated around 4 am. Unsettled conditions are forecast to continue from Sunday night into Monday night. The frontal boundary is expected to remain in our region on Sunday night with another low expected to pass through our area on Monday. We are anticipating the likelihood of showers and thunderstorms associated with another convective complex on Sunday night. Precipitable water values should be in excess of 2 inches across the central and southern parts of our forecast area. As a result, we will mention the potential for heavy rain at that time. The chance of showers and thunderstorms will continue into Monday and Monday night with the passage of the low. The low is expected to begin pulling the front to our southeast and south on Monday night. The axis of a mid level short wave trough approaching from the northwest is anticipated to pass overhead on Tuesday afternoon and it will be followed by a shot of dry weather for the mid week period. A cold front is expected to approach from the northwest on Thursday and it should pass through our region from Thursday night into early Friday. We will mention a chance of showers and thunderstorms with the front. Dry weather is forecast to return for next weekend. Temperatures are expected to be seasonable during the new week with no excessive heat events in sight. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Sub-VFR CIGs (with locally patchy fog as well) this morning generally along and north of the Mason-Dixon Line should slowly improve to VFR in the 12Z-15Z time frame with northeast winds 5-10 kts. Thereafter, winds may become easterly or southeasterly (or even transition to southwesterly this afternoon for a time), remaining around 5-10 kts. Stray showers may occur this morning, but best chances for precipitation are after 00Z, where scattered to widespread strong thunderstorms are expected to move through the area. Erratic winds/gusts are likely with any shower/storm. OUTLOOK... Sunday night...MVFR and IFR conditions in showers and thunderstorms. The rain may become heavy. Monday...A chance of showers and thunderstorms with conditions improving to VFR outside of any areas of rain. Monday night...Conditions lowering to MVFR and possibly IFR. A chance of showers and thunderstorms. Tuesday...MVFR and IFR conditions in the morning improving to VFR. Tuesday night through Thursday...Mainly VFR. && .MARINE... Sub-advisory conditions are expected to continue through tonight. Storms are in the forecast again tonight, and some of these may be strong and will likely produce heavy rainfall. Locally rough seas should be expected near precipitation. Outside of any precipitation, seas will generally be 2-4 feet with a northeast to east flow developing this morning veering to the southeast tonight, generally 5-15 kts. OUTLOOK... Sunday night through Thursday...No marine headlines are anticipated. RIP CURRENTS... A moderate risk of rip currents is expected to continue through today as the new moon combined with an underlying swell of 10-12 seconds is occasionally showing up in buoy observations (though less so overnight so far). && .TIDES/COASTAL FLOODING... We have issued another coastal flood advisory for this evening`s high tide for the Atlantic coastal waters and for adjacent portions of the coast of Delaware Bay, as the new moon combined with increased onshore flow today should provide conditions at least as favorable, if not somewhat more so, for minor coastal flooding. With last evening`s high tide, some minor flooding occurred in most of this area (though generally below advisory levels). Thus, with the somewhat higher forecasts observed with this evening`s high tide, an advisory seems warranted. && .CLIMATE... ABE 1.29" today as of 03/23. Daily record is 1.54 set in 1938. POR 1922. ABE mo total as of 01z/23 is 6.46",20th wettest in the POR dating back to 1922. July record 10.42" 1969. && .EQUIPMENT... Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area. Hibernia PA and Hamburg NJ transmitters are off the air (lost Hibernia this evening around 0015z/23). Both have trouble tickets. No known rts. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for DEZ002>004. MD...None. MARINE...None. && $$ Synopsis...Iovino Near Term...CMS Short Term...CMS Long Term...Iovino Aviation...CMS/Iovino Marine...CMS/Iovino Tides/Coastal Flooding...CMS Climate...Drag Equipment...Staff

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